What condition describes the appearance of darkened nail beds due to excess melanin?

Melanonychia is the condition characterized by darkened nail beds resulting from an excess of melanin. This pigmentation can arise from various factors, including benign conditions such as melanoma, trauma, or certain medications, as well as systemic diseases. The presence of melanin gives the nails a brown or black appearance, distinguishing melanonychia from other nail disorders.

In contrast, leukonychia refers to white spots or lines on the nails, often due to trauma or mineral deficiencies. Onychomycosis is a fungal infection that usually causes discoloration, thickening, and separation of the nail from the nail bed, but does not typically involve increased melanin. Onychorrhexis describes the condition of brittle nails that may have vertical ridges or split, which is unrelated to changes in pigmentation. Thus, melanonychia is the specific term for darkened nail beds due to melanin, making it the correct choice in this scenario.